Isomorphic coordination polymers of cobalt(ii), zinc(ii) with a flexible ligand of cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid and their molecular alloys: crystal structures, thermal decomposition mechanisms and magnetic properties

Abstract

Three new isomorphic coordination polymers of Co²⁺, Zn²⁺ ions with flexible multicarboxylic acid ligand of the cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid (H₄L), [Co₄L₂(H₂O)₈]·3H₂O (1), [Zn₄L₂(H₂O)₈]·3H₂O (2) and [Co_0.8Zn_3.2L₂(H₂O)₈]·3H₂O (3), have been synthesized under hydrothermal conditions and by means of controlling the pH of the reaction mixtures (with an initial pH of 6.0 for 1, 4.0 for 2, and 5.0 for 3, respectively). In the crystal of 1, two crystallographically different Co²⁺ ions (Co1 and Co2) form a negatively-charged coordination polymeric chain, which contains a centrosymmetric, linear, trinuclear Co²⁺ cluster (Co₃L₂) subunit; another crystallographically independent Co²⁺ ion (Co3) coordinated to six water molecules acts as a counter ions to link the neighboring coordination polymeric chains via intermolecular H-bond interactions. The Co²⁺ ions in 1 were completely and partially replaced by Zn²⁺ ions to give 2 and 3, respectively. Complex 3 shows a novel molecular alloy nature, due to the random distributions of the Co²⁺ and Zn²⁺ ions. Three isomorphic complexes exhibit distinct thermal decomposition mechanisms. The deprotonated cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid ligands decompose at 420–750 °C to give the residue CoO in 1, ZnO + C in 2 and CoO + ZnO in 3. Complex 1 shows a complicated magnetic behavior with co-existence of antiferromagnetic exchange interactions between neighboring Co²⁺ ions as well as strong spin–orbital coupling interactions for each Co²⁺ ion; complex 3 exhibits a magnetically isolated high-spin Co²⁺ ion behavior with strong spin–orbital coupling interactions.